In general, a level shifter is used for shifting a predetermined voltage level of an input signal into another voltage level. That is, a level shifter supplies a low voltage level signal to a high voltage level signal by shifting a low voltage level input signal into a high voltage level output signal. Alternatively, a level shifter may supply a high voltage level signal to a low voltage level signal by shifting a high voltage level input signal into a low voltage level output signal.
FIG. 1 shows a diagram of a configuration of a conventional level shift circuit.
As shown in FIG. 1, the level shift circuit is a cross-coupled circuit, and includes two p-type transistors P1 and P2, and two n-type transistors N1 and N2.
When a high level input signal in1 is inputted to the gate of transistor N1 and a low level input signal in2 is inputted to the gate of transistor N2, node A becomes a reference potential (e.g., a ground potential) because the transistor N1 is turned on and the transistor N2 is turned off. When node A becomes ground potential, transistor P2 is turned on because the ground potential is applied to the gate of transistor P2, and an output signal out2 of a supply voltage VDDH is outputted because the supply voltage VDDH is supplied to a node B.
In addition, when a low level input signal in1 is inputted to the gate of transistor N1 and a high level input signal in2 is inputted to the gate of transistor N2, node B becomes the reference potential (e.g., a ground potential) because transistor N1 is turned off and transistor N2 is turned on. When node B becomes the ground potential, transistor P1 is turned on because the ground potential is applied to the gate of transistor P1, and an output signal out1 of the supply voltage VDDH is outputted because the supply voltage is supplied to node A.
Such a conventional level shifter uses n-type transistors N1 and N2 as driving transistors. However, an n-type transistor has a higher threshold voltage and less mobility compared to a p-type transistor. For example, in the level shifter shown in FIG. 1, when a threshold voltage of the n-type transistors N1 and N2 is close to a voltage of the input signals in1 and in2, the n-type transistors N1 and N2 are weakly turned on. For example, when the threshold voltage of the n-type transistors N1 and N2 is 3V and the voltage of the input signal is 3.3V, Vgs−Vth of the n-type transistors N1 and N2 is only 0.3V, where Vgs denotes a voltage between the gate and the source of a transistor, and Vth denotes a threshold voltage. Therefore, in this case, the operation of the n-type transistor is unstable and its operating speed is slow because the n-type transistor is weakly turned on. Accordingly, it is difficult to apply the n-type transistor to a display device operating at a high speed.
Specifically, the level shifter may be integrated on a glass substrate of a display panel by forming the level shifting by a thin film transistor (TFT) using a low temperature polysilicon (LTPS). The LTPS TFT has less mobility and a higher threshold voltage compared to a metal oxide semiconductor (MOS) transistor using single crystalline silicon. Accordingly, a level shifter using LTPS TFT has difficulty achieving a high operating speed and therefore it is difficult to apply such to a display device operating at a high speed. Even if the operating speed is accelerated by increasing the voltage of the input signal, the level shifter using LTPS TFT is difficult to be applied to a display device, which requires low power consumption, since power consumption is increased by increasing the voltage of the input signal.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and therefore, unless explicitly described to the contrary, it should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known in this country to a person of ordinary skill in the art.